Carpal tunnel syndrome is at present the most widespread occupational health hazard in the industrial world. Many billions of dollars are being consumed each year in lost working time and in the diagnosis and treatment of this syndrome.
Although some of the physiological factors associated with idiopathic carpal tunnel syndrome are well documented, including a non-inflammatory increase in the volume of synovium resulting in an increase of pressure in the carpal tunnel contents, the etiology of the change in quantity of the synovium has remained obscure.
Tendons in general have high longitudinal strength and rigidity, but are soft compliant tissues in the transverse direction, much like a bundle of fine wires, which can be easily molded to different cross-sectional shapes, even under tension longitudinally. A synovium wraps around, nourishes and lubricates the nine tendons of the extrinsic muscle tendon units which pass through the carpal tunnel. Each finger of the hand utilizes two of these nine tendons, with the thumb utilizing the last, to flex the joints of the digits. The median nerve also passes through the carpal tunnel to enervate certain muscles and provide sensation to related areas of the hand. Any increase in volume of the carpal tunnel contents, either synovium or tendons, results in compression of this nerve with secondary signs and symptoms of carpal tunnel syndrome.
In anatomic areas other than the carpal tunnel the biomechanical effect of each tendon is defined by discrete fiberosseous tunnels, effectively forming pulleys, that dictate the precise offset and location of the tendon as it crosses the joint to generate the rotational forces or moments needed to effect controlled angulation and stability of each joint. A pulley with a precise moment arm is also required in the carpal tunnel for each tendon as it crosses the carpus. Due to the compliant nature of the synovium, these pulleys can only be formed and maintained in the carpal tunnel, for all motions of the carpus, by a synergistic tensioning of all nine tendons of the carpal tunnel to form "dynamic soft tissue pulleys" for each tendon. Without this dynamic action, a singular contraction of a muscle tendon unit would cause that tendon to translate past the unresisting tendons in a snapping action until it rested against the more rigid wall of the carpal tunnel. The moment arm required for precise control would be lost for that particular motion. Such singular contractions can occur in individuals using repetitive motions such as typing, knitting, or peeling of vegetables.
The brain normally orchestrates the simultaneous activation of all the proper muscle tendon units, but with continued, often boring or tiring, repetitive motions will economize or neglect to synergistically contract the needed muscle tendon units requisite to stabilize the tendon or tendons actually doing the work.
Normal synovium is a filmy, compliant tissue which is incapable of resisting any translational repositioning of the tendons. In the presence of unstable carpal tunnel tendons, the synovium is subjected to repetitive shear and other stresses that result in its thickening and loss of elastic compliance. The thickening which occurs over time, increases the pressure in the limited volume of the carpal tunnel, with the secondary compromise of the blood supply to the median nerve and secondary symptoms of carpal tunnel syndrome.
There has not heretofore been provided a method or apparatus to monitor the motions of the tendons in the carpal tunnel and distinguish whether they are sliding properly in "soft tissue pulleys" or are snapping past each other in such a way as to injure the synovium and predispose the individual to the occurrence of carpal tunnel syndrome.